ATLANTA, Georgia (CNN) --John Walker Lindh told military and FBI questioners he believed the September 11 attacks were the first of three waves of terrorist strikes against the United States, according to secret documents obtained by CNN.

Walker Lindh, the first American taken prisoner in Afghanistan as a Taliban fighter -- and scheduled to be sentenced Friday -- also said he turned down an offer to take part in suicide attacks against the United States, and that he believed as many as 50 operatives had been sent on missions against the United States and Israel.

The secret documents are summaries of his first interrogations by U.S. Special Forces troops on December 1, 2001, and of three interrogations conducted by FBI agents in Afghanistan on December 9 and 10.

The FBI's interrogation report says Walker Lindh related that after September 11, one of his former al Qaeda training-camp instructors said "that UBL (Osama bin Laden) said this was the first attack. ... The group speculated that the second attack would involve attacking nuclear facilities, oil/gas pipe lines, or some kind of biological attack."

Walker Lindh's lawyers had sought to suppress these documents as evidence, saying they were made under duress, but that was before they reached a plea bargain with federal prosecutors in July. In that bargain, Walker Lindh agreed to plead guilty to charges that he supplied services to the Taliban and carried an explosive during the commission of a felony.

Walker Lindh is expected to be sentenced to 20 years in prison. CNN has learned that he plans to address the court and the American people before he is sentenced. (Full story)

He was among a group of Taliban soldiers who surrendered to the Northern Alliance in late November. They were taken to Mazar-e Sharif and questioned by two CIA employees. (More on his capture)

When many of the prisoners staged a revolt, Walker Lindh, who was wounded in the leg, went into hiding and surrendered again a week later.

Walker Lindh was on the front lines, with a Taliban unit, when the September 11 attacks took place. His military questioners write: "Source showed remorse and signs of regret" when he was asked about the attacks.

He told his interrogators that one of his former instructors said that "this was the first attack" and that a second wave would come at the beginning of Ramadan, in mid-November, and "make America forget about the first attack." The instructor also talked of a third wave, in early 2002, but provided no details.

While Walker Lindh told his Special Forces interrogators that the second phase of attacks could involve biological weapons or attacks on nuclear weapons facilities, there is a comment from the questioners saying "source was making assumptions and conjectures based on talk among his colleagues."

Lindh described UBL (Osama bin Laden) as being approximately 6'4", well built, sits down a lot, drinks a lot of water, tires easily, sometimes stops talking and excuses himself, mild mannered, and a quiet speaker.

-- FBI interrogation report, December 9-10, 2001

Walker Lindh repeated this account to the FBI, when it questioned him.

California to Afghanistan

Walker Lindh told the FBI that his interest in Islam began when he saw the 1992 film "Malcolm X" when he was 12. That interest launched an odyssey that took him from being a teenage Muslim convert in northern California to an Arabic language school in Yemen, and on to Pakistan and Afghanistan.

In the summer of 2001, according to interrogation reports, Walker Lindh was at an al Qaeda-run camp, al Farooq, for a seven-week training course.

He told interrogators that although he wanted to join the Taliban, he was sent to al Farooq because, "The Arab group is Osama bin Laden's al Qaeda's group ... and that was the only way to get to the front lines."

Using the abbreviation "UBL" to refer to bin Laden, the FBI report continues: "Lindh knew UBL's/al Qaeda's purpose was to fight Americans" before he went to the camp.

Walker Lindh outlined a training course at the camp that included:

Three weeks of familiarization with weapons.

One week's study of topography and maps.

One week of battlefield training.

The shooting course includes practicing assassinations with pistols and rifles and shooting from motorcycles and cars. Civilian warfare courses include terrorism, forgery of passports and documents, poisons, mini-explosions, and an intelligence course which teaches trainees how to avoid detection by police.

-- U.S. Special Forces interrogation report, December 1, 2001

One week of instruction in explosives.

Walker Lindh told his FBI interrogators that bin Laden visited the camp three to five times while he was there, usually with one of his sons. He said he met bin Laden once, for five minutes, with other recruits and that bin Laden "made small talk and thanked them all for taking part in the jihad." (More about meeting bin Laden)

Walker Lindh also told the FBI that the head of all al Qaeda's training camps personally asked him if he would take part in missions against the United States and Israel.

He said he declined, and he also turned down a chance to swear allegiance to al Qaeda. Instead, says the report, "He swore allegiance to Jihad."

During these initial interrogations Walker Lindh did not mention any other Americans being at al Farooq, although several men from Buffalo, New York, would later be arrested and charged with being at the camp at the same time. (Full story)

Even before he left al Farooq, Walker Lindh said one of his instructors told him that bin Laden "sent 50 people to carry out 20 ... suicide operations" and that the group believed the attacks were aimed at the United States and Israel.

At the end of his questioning by the military, John Walker Lindh shared a final thought about Osama bin Laden's whereabouts.

Walker Lindh said families of al Qaeda members had been moved from Afghanistan to Yemen during 2001 ... and that he thought "Osama bin Laden may be planning on moving there."